High-power transistors are commonly used as load switches for a wide variety of applications. A high-side load switch is one that it is controlled by an external enable signal, and connects or disconnects a power source to a given load. A low-side load switch, on the other hand, connects or disconnects the load to g round, and therefore sinks current from the load.
When a transistor is switched on or off, it does not immediately switch from a non-conducting to a conducting state; and may transiently support both high voltage and high current. Consequently, when current is applied to the gate of a transistor to cause it to switch, a certain amount of heat is generated which can, in some cases, be sufficient to destroy the transistor. Therefore, it is necessary to keep the switching time as short as possible, so as to minimize switching losses. Typical switching times are in the range of microseconds. The switching time of a transistor is inversely proportional to the amount of current used to charge the gate. Therefore, switching currents are often required in the range of several hundred milliamperes, or even in the range of amperes. For typical gate voltages of approximately 10-15V, several watts of power may be required to drive the switch. When large currents are switched at high frequencies, (e.g. in DC-to-DC converters of large electric motors), multiple transistors are sometimes provided in parallel, so as to provide sufficiently high switching currents and switching power.
Switching signals for a high-power transistors are usually generated by a logic circuit or a microcontroller, which provides an output signal that typically is limited to a few milliamperes of current. Because high-power transistors generally require drive inputs having current demands far greater than those provided by low voltage logic signals generated by logic circuits and microcontrollers, gate drivers must be provided to control high-power transistors. A gate driver is a power amplifier that accepts a low-power input from a controller IC and produces a high-current drive input for the gate of a high-power transistor such as an insulated-gate bipolar transistor (“IGBT”) or power metal-oxide-semiconductor field-effect transistor (“MOSFET”). Gate drivers can be provided either on-chip or as a discrete module.
In summary, a gate driver's primary function is to deliver the correct amount of charge to the power switch quickly. An important secondary function of a gate driver is to act as a signal translator, by converting low level on/off signals from a low power circuit provided, for example, by a microprocessor, to higher power levels required to operate the power switch. Gate drivers are offered in various input configurations and power levels to operate different types and sizes of power switches. Larger power switches require gate drivers with larger driving capability. In addition to the above capabilities, high-voltage gate drivers allow circuits to operate in off-line environments. Applications for high-voltage gate drivers consist of motor control circuits used in appliances as well as industrial drives, off line switching power supplies, un-interruptible power sources, high-end adapters, lighting circuits (CCFL, CFL, LED) and many more.
High-side integrated gate drivers are used in applications requiring the switching of power MOSFET and/or IGBTs when used as high-side switches. FIG. 1 shows a load 101 having a path 102 to ground (the low side) and a connection to a high-voltage power supply 103 through a high-side switch 104. A pulsating output at terminal 7 of a controller chip 105, which is controlled by a square wave pulse generator 106 input at terminal 2, charges the gate 107 of high-side switch 104 (in this particular case, an N-channel enhancement-mode, insulated-gate, field effect transistor, or IGFET) through a resistor 108. The controller chip 105 is configured so that node 109 is connected to ground via a current path between terminals 3 and 6 when the load 101 is disconnected from the high-voltage power supply 104 (i.e., when the channel of high-side switch 104 is not conducting). The battery 110 provides a voltage to charge the gate 107 of high-side switch 107 to the required level.
IGFETs, or MOSFETs as they are frequently called, are invariably selected as gate drivers because, in contrast to bipolar transistors, MOSFETs do not require constant power input to remain in a conducting or non-conducting state. The insulated gate-electrode of the MOSFET functions as a capacitor, which must be charged or discharged each time the MOSFET is switched on or off. As such a transistor requires a particular gate voltage in order to switch on, the gate capacitor must be charged to at least the required gate voltage for the transistor to be switched on. Similarly, to switch the transistor off, this charge on the gate capacitor must be dissipated. Thus a MOSFET gate driver requires the application of control power only during the short time that it takes to charge the gate electrode to an optimum threshold voltage that is required for conduction to be induced in the channel of the MOSFET. The gate drive voltage is turned on for a period of time determined by the system duty cycle. The duty cycle period is of sufficient length so that the insulated gate of the driver transistor will be charged to the optimum threshold voltage. Once the gate capacitor is charged, it draws no further power, and the transistor remains in an ON state until charge on the gate is dissipated. Because gate drive voltage is typically a few volts above the power rail voltage, a separate power source must be available to provide the energy to charge the gate of the high-side driver sufficiently to render it conductive. This supply is referred to as a “bootstrap supply”. If the bootstrap supply is of a dc-dc converter type, then the high-side switch can be maintained on indefinitely. When used to control current flow in a three-phase brushless DC motor, the high-side switch must be turned on and off for each revolution of the motor. Thus, the capacitor must be recharged after it charge is applied to the gate electrode of the high-side switch. In FIG. 2, a charging path 201 and a low-side switch block 204 have been added to the schematic diagram of FIG. 1. It should be understood that whenever high-side switch 104 is conducting, the low-side switch is not. The low-side switch transistor 205 is controlled by the pulses generated by low-side pulse generator 206. Those pulses are passed to the insulated gate 207 of low-side switch transistor 205 through an amplifier 208 and a resistor 209.
Still referring to FIG. 2, the bootstrap charging path couples VDD to both terminal 8 of the controller chip 105 and to the positive plate of a bootstrap capacitor 203. The presence of a diode 202 in the charging path 201 prevents the discharge of the capacitor 203 back to VDD. The negative plate of capacitor 203 is coupled to the load 101 and to terminal 6 of the controller chip 105. In response to the rising edge of each pulse from high-side square wave pulse generator 106 received at terminal 2, the controller chip 105 interconnects terminals 7 and 8, thereby dumping the charge on capacitor 203 to the insulated gate of the high-side switch 104. After a set time sufficient for the gate 107 to charge to its optimum level, the controller chip 105 breaks the connection between terminals 7 and 8. The gate electrode 107 remains charged. When the trailing edge of each pulse from the pulse generator 106 causes the controller chip 105 to short terminal 7 to ground terminal 3 and dissipate the charge on insulated gate 107, conduction through the high-side switch 104 stops. Node 109 is maintained at ground level through low-side switch transistor 205 while the capacitor 203 charges. When the charge on gate 207 goes low, low-side switch transistor 205 stops conducting and node 109 becomes isolated. The cycle begins anew as the charge on capacitor 203 is again sent to the insulated gate 107 of high-side switch transistor 104.
Typical application for high-side gate drivers include, but are not limited to, appliance motor drivers, industrial motor controllers, lighting systems and multiple phase power supplies. Since these types of gate drivers are used in high voltage and high power systems, protection of the external system as well as maintaining a safe environment are of utmost importance.
Three-phase alternating current induction motors were developed independently by Galileo Ferraris, Mikhail Dolivo-Dobrovolsky and Nikola Tesla in the late 1880s. In a balanced three-phase power supply system (by far, the most common type), three conductors each carry an alternating current of the same frequency and voltage relative to a common reference (such a reference is typically connected to ground and often to a current-carrying conductor called the neutral) but with a phase difference of one third the period; hence the voltage on any conductor reaches its peak at one third of a cycle after one of the other conductors and one third of a cycle before the third conductor. From any of the three conductors, the peak voltage on the other two conductors is delayed by one third and two thirds of one cycle respectively. This phase delay gives constant power transfer over each cycle, and results in a rotating magnetic field in an electric motor's stator. In an induction motor, electric current in the rotor windings needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. So that induction can occur, an induction motor's rotor rotates about 95 percent of synchronous speed (a five percent slip rate from sychronicity). A three-phase synchronous motor does not rely on slippage, but employs permanent magnets mounted on the rotor. Three-phase motors are known for their high efficiency (as high as 95 percent) and small size.
For small-motor applications many appliance designers favor modern three-phase brushless DC (BLDC) motors because of their high efficiency and small size per horsepower of output. A BLDC motor can, indeed, be 3-phase, which refers to the number of electrical phase windings on the motor. Power inputs are switched via a three-phase power “bridge” from a DC voltage supply. This “bridge” consists of 6 power devices, which are generally IGBTs, but sometimes MOSFETs. Square wave power pulses are delivered to each of the three phase windings on the stator. Pulse-width modulation is used to control the power output, and the order in which the three motor phases are energized determines the direction of rotor rotation. In order to ensure safe operation under any load condition, designers must carefully design control logic to address torque, speed control, and power-delivery issues.